feat: Document the $sync namespace

[gmaclennan]

Adds up-to-date documentation for the $sync namespace. @ErikSin.
Note that this documents the API as-of digidem/comapeo-core#411, so that needs to be merged and released first.
I think this is potentially missing information that might be needed to implement all the screen designs we have: the sync state includes the number of connected peers, but it does not include the number of peers that are syncing the data cores.
I also think we need better vocabulary for talking about the different phases of sync: internally our data is divided into five "namespaces", and each namespace contained different record types:

• auth namespace contains the role and coreOwnership record types, which establish membership of a project.
• config namespace contains the deviceInfo, preset, field, icon and projectSettings record types, which is the data needed to start collecting data (we call it "project metadata")
• blobIndex namespace contains information about all the blobs in the project e.g. filenames and file sizes, like a directory listing.
• data namespace contains the observation record type (and in the future will contain node, way and track data types).
• blob namespace contains the raw photo, video and audio data (media data)

We split sync into two parts:

• initial sync of the auth, config and blobIndex namespaces, which also shares information about how many observations and blobs need to be synced.
• data sync of the data and blob namespaces.

initial sync happens automatically and in the background whenever a project member connects on the local network. data sync requires user interaction and is started via $sync.start().

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Name	Status	Preview	Comments	Updated (UTC)
mapeo-core-docs	❌ Failed (Inspect)			Dec 11, 2023 4:30pm

[achou11]

Couple of small corrections but otherwise lgtm